Tomographic images obtained by scanning bodies with high energy radiation and detecting absorption by a scintillating crystal detector have not been completely sharp and clear, especially at the edges or boundaries. Heretofore, attempts have been made to improve image definition, especially at the boundaries, by surrounding the scanning body by a bag of water or employing a shutter to cut off the radiation when the scanning beam leaves the edge of the body being scanned. It was known that a portion of the incident radiation on a scintillation detector is converted into phosphorescent radiation of approximately the same wavelength as the flourescent radiation and this might be problematical. For a telluriumdoped sodium iodide scintillation crystal, 91 percent of the absorbed radiation is converted into fluorescent radiation and 9 percent is converted into phosphorescent radiation. (S. Koicki, A. Koicki and V. Ajdacic "The Investigation of the 0.15 s Phosphorescent Component of NaI(TI) and Its Application in Scintillation Counting, " Nuclear Instruments and Methods, 108: 198-9 (1973)). The production of phosphorescent radiation interferes with the measurement of the intensity of the high-energy radiation. According to the Koicki et al. article, the fluorescent decay time for a tellurium-doped sodium iodide crystal of 225 nanoseconds compares to a phosphorescent decay time of 150 milliseconds. When photons strike the detector with relatively high frequency, the phosphorescence produced by photons prior in time will persist and interfere with the detection and resolution of the more-or-less instantaneous flourescent events.
When the flourescent and phosphorescent radiation are of about the same frequency and wavelength, a photomultiplier tube (PMT) positioned to detect the lower-energy radiation produced by the scintillator cannot readily distinguish the flourescent event from the phosphorescent event. The electrical output signal of the PMT is, therefore, the summation of the flourescence produced by the immediate photons and the phosphorescence produced by the prior photons.
It was, however, not therefore understood or appreciated that phosphorescent afterglow is the primary cause of poor image definition of tomographic images, especially at the boundaries. An object of this invention is, therefore, to provide a simple and economical means for improving the definition of a scintillation detector, and, more particularly, for sharpening the definition of tomographic images obtained from tomographic scanning and scintillating counter detection.